Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/326

 A different way of inferring the necessity for light-pressure was indicated in 1876 by A. Bartoli, who showed that, when radiant energy is transported from a cold body to a hot one by means of a moving mirror, the second law of thermodynamics would be violated unless a pressure were exerted on the mirror by the light.

The thermodynamical ideas introduced into the subject by Bartoli have proved very fruitful. If a hollow vessel be at a definite temperature, the aether within the vessel must be full of radiation crossing from one side to the other: and hence the aether, when in radiative equilibrium with matter at a given temperature, is the seat of a definite quantity of energy per unit volume.

If U denote this energy per unit volume, and P the light-pressure on unit area of a surface exposed to the radiation, we may apply the equation of available energy

Since, as we have seen, this equation gives and therefore U must be proportional to T4. From this it may be inferred that the intensity of emission of radiant energy by a body at temperature T is proportional to the fourth power of the absolute temperature—a law which was first discovered experimentally by Stefan in 1879.

In the year in which Maxwell's treatise was published, Sir William Crookes obtained experimental evidence of a pressure accompanying the incidence of light; but this was